Building the Stinson SR-9 (16)

Here’s the left LG strut with the plywood outer fairing skin in place. The fairings on the inside and outside faces of each strut are the same.

This is a front view of the right strut. You can see where I have epoxied a strip of 1/4″ x 1/2″ basswood along the leading edge and sanded it to blend with the rest of the surface.

Now we can get started on the cowl. The nose bowl and extension section have both been cut from the molded plastic matrix sheets and then trimmed and sanded to fit neatly together. I have left the front section of the nose bowl uncut for now…more on that soon.

I have built up the cowl ring from the various die-cut plywood forward and aft cowl ring sections. Here I’m slipping it back over the motor and mounting box to rest against the fuselage front/firewall.

Here’s where the results of careful fitting pay off. The cowl mounting base fits snugly around the motor box with its mounting tabs aligned to bolt it independently to the firewall. Notice that the outer circumference fits with an offset just equal to the thickness of the cowl.

This is why I left the plastic front of the nose bowl in place. I marked the exact center of the circumference and cut a hole to slip sungly over the shaft of the AXI 5320 motor. With that done the cowl, by default, is centered exactly on the airplane’s centerline as defined by the motor shaft. I used a scrap of balsa as a gauge to align the outer edges of the cowl with the shaft.

In spite of all my efforts, the fit of the 1/8″ balsa sheet nose skins to the rear edge of the cowl wasn’t perfect. I sanded away the high spots, slipped the assembled cowl into place with a sheet of plastic wrap inside to protect the nose sheeting, and used SLOW (thick) ZAP along the rear edge of the cowl to create an accurate, secure mounting base.

After letting the ZAP cure hard, I used a coarse sanding block (80 grit production paper) to smooth off the outer contour of the joint between the cowl and the nose. Since I made sure the ZAP formed a shallow lip around the rear circumference of the cowl, this finished joint creates a precise fit that permits me to remove the cowl if I need to and easily slip it back into the right position.

For the next step I pulled the cowl off and set it aside to work on the dummy engine and its mount. As you’ll see, since that part of the assembly determines exactly where the rocker box blisters will go, the dummy engine has to be located before we can go any further. I’m going to use a 1/5 scale generic 9-cylinder radial engine resin casting from Frank Tiano Enterprises. The relationship of the full scale engine to the cowl determines exactly how far fore-and-aft the model engine needs to be mounted. With that figured out I adjusted the dimensions of the 1/5 scale crankcase so it would fit the cowl and the AXI 5320 motor, then worked out where the rear face of the dummy would need to be for all that to work. That determined the exact position of the “picture frame” mount around the rear end of the motor. The inside of the dummy engine crankcase is opened up for about 1/8″ of clearance around the AXI motor. This 1/8″ thick wrap of masking tape ensures that the one centers directly on the other.

Here’s the as-yet-unfinished dummy radial in position over the tape spacer.

A couple of hex-head sheet metal screws in pre-drilled holes take care of mounting the dummy engine. The sequence of assembly of the finished model will guarantee that I’ll have access to these screws when I need it.

Now for the messy part. The molded cowl MUST be cut out to fit over each of the eighteen dummy rocker box covers (unless I want to cut back the dummy cylinder heads inside and compromise the appearance of the finished airplane). Since this is not an exact scale model…with no precise drawings to work from… it made the most sense to pencil mark the cutout locations one at a time, cut them out with a rotary burr on the drill press, then go back and adjust and correct and cut one more at a time until I got ’em all right.

Take your time, cut cautiously, and when you’re done it ought to look like this. (Sorry about the focus.)

The rocker box blisters are supplied as individual moldings, each within its own matrix sheet. All eighteen must be trimmed to shape…one at a time.

The rocker box blisters go on like this. You’ll need to do some careful trimming and sanding to get a close fit, but there’s no other way I know of to get it right.

No matter how carefully you fit those blister moldings there will be little gaps and crevices everywhere…AND…
there’s supposed to be a wide radius fillet around the base of each one. I used Stits SuperFil 2-part epoxy putty (the blue stuff) to build up the required fillets. Here I’m beginning the job of sanding it out using 100-grit production paper DRY so I can see exactly how the abrasive is cutting.

You don’t need me to tell you that sanding epoxy putty DRY creates lots of dust. I switched over to 220 grit wet-or-dry with lots of water to finish the job. There is no way you can rush this part…it’s going to take at least a couple of hours, but the results of getting it right are worth it.

Here’s the fully sanded out cowl fitted in place on the airplane. NOTE: I worked on the windshield while you weren’t here…but…I’ll devote a whole installment to that part of the job soon.

While the cowl was back on the airplane I very carefully marked a centerline along the top and bottom. This is where the cowl splits apart on the full scale Stinson. A nice, sharp razor saw keeps the job of cutting the two halves apart under control.

What you didn’t get to see before was my putting those little corner fillets of SuperFil into six spots along the top and bottom along the centerline inside the cowl. Inside each fillet is a carefully positioned length of 1/8″ I.D. aluminum tube at right angles to the plane on which the cowl splits. When I made the cut, the open end of each half-section of tube appeared in exact orientation to its opposite half. I’ll add steel wire locating pins to each pair of holes to ensure alignment of the completed cowl.

During the initial assembly I left the cowl mounting ring attached to what would become the left half. When I cut the cowl in half I split the left and right halves of the mounting ring, too. Now I’ve bolted the left ring with its attached cowl section back into place on the firewall, then added the right half of the mounting ring without the cowl half. I’m ZAP’ing them together with a couple of semi-circular 1/8″ plywood reinforcements. The plastic wrap behind the mounting ring will keep the adhesive where I want it after I snug up all four cowl ring mounting bolts.

I added a flange of 1/16″ plywood along the inside of each of the parting edges and backed up several points on each one with little blocks of 1/4″ thick balsa. I then cut three shallow 1/2″ diameter holes through the top and bottom plywood flanges into the blocks and fitted a 1/2″ rare earth magnet into each hole so the outer surface of each magnet will lie flush with the plywood.

I seated each of the magnets in the left half of the cowl into epoxy and let it cure hard.

With the right half of the cowl slipped back into place I marked the location for each of its magnets.

Prior to taking this shot I cut oversize holes for each of the magnets in the right cowl half to allow space for each to move as needed to mate up exactly with their opposites. I’ve turned the entire fuselage onto its side to create a level working plane for the next part of the job so the wet epoxy will stay where I want it. With more plastic wrap in place to keep the epoxy under control, I dropped the right side magnets into place, letting them sort out their polarity and get centered.

My old artist’s pallette knife is still a good tool for applying epoxy. I’m using 15-minute ZAP here.

When I fitted the right cowl half into place, lining up each of the guide pins into its respective guide tube and ensuring that each new magnet seated properly in its hole, I snugged both halves tightly into place with masking tape and left it overnight to cure. When I opened it up the next morning and cleaned up the plastic wrap and epoxy “squeezeout”, this is what I had.

Here’s the finished cowl all ready for priming and sanding prior to painting. Later on I’ll demonstrate how easily I can remove and replace it to install battery packs and so on.

Sometimes I just gotta’ go off on my own and work without an audience. This is the scale cooling air exit I built per full scale SR9 drawings and photos. I’ll give you a better look at this later on as well.